1. Field of the Invention
This invention relates generally to voice-switched telephone amplifier circuits of the type especially used in repeaters, speakerphones, and handicap-type amplifiers, and more specifically, to the type of amplifier which is adapted to be disposed entirely within a telephone station set or handset.
2. Description of the Prior Art
Telephone subscribers many miles from the central office require special equipment to compensate for the poorer transmission performance caused by the inherent losses in the telephone line. The conventional approach has been to provide increased gain in the central office. The amount of gain that can be added at the central office is limited by cross-talk considerations. Loss in some loops cannot be compensated for by gain devices located in the central office. A different approach has been to add auxiliary repeaters to the line at locations remote from the central office which compensate for the attenuation of the telephone signals along the telephone line. If only a few subscribers are to be connected in an area the added cost of a series of auxiliary repeaters is generally high.
The amplifier circuit in accordance with the present invention utilizes a more practical approach, i.e., extra amplification is provided at the subscribers station rather than at an exposed location out in the loop. Not only has this approach proven to be substantially less expensive initially, but maintenance expenses are lower, and the equipment is within the protected environment of the subscriber's house.
In addition to compensating for the electrical losses in the telephone line, another reason for providing extra amplification at the telephone set is to compensate for a weak acoustic signal applied to the telephone set. For example, compensation must be provided when using a speakerphone, or similarly, when an individual subscriber has hearing or speech disabilities.
Examples of prior art auxiliary amplifiers adapted to be located at the subscriber station, and more particularly within the telephone handset, are shown by U.S. Pat. No. 2,842,623, issued to Philip N. Lehr on July 8, 1958 and U.S. Pat. No. 3,254,160, issued to P. B. Day and J. Radomski on May 31, 1966. These patents show supplemental handicap-type amplifiers which are adapted to be disposed within the telephone handset and which supply increased gain to the receive signal or to both the transmit and the receive signals.
The problem with merely adding transmit and receive gain in the telephone set is that it causes the sidetone signal which is heard in the receiver to increase by the sum of the transmit and receive gains. If the sidetone signal increases to an objectionable level, it becomes annoying to the user; moreover, there is a tendency for the user to lower his voice, thus defeating the purpose of the additional auxiliary amplification.
Voice-switching is one possible solution. Voice-switching loss (or gain) to provide sidetone control or automatic gain control is shown in the telephone amplifier art by U.S. Pat. No. 3,602,648, issued to R.E. Holtz and J.A. Markevich on Aug. 31, 1971 and U.S. Pat. No. 3,823,273, issued to R.H. Beeman and R.T. Cleary on July 9, 1974. These particular patents are examples of voice-switched telephone amplifier circuits which compensate for the inherent loop losses in the line to a distant location. The prior art, however, has not disclosed a stable line-powered telephone amplifier circuit which can operate with a conventional hybrid network on loop currents as low as 3 milliamperes to provide telephone sets with additional transmit and receive gain while automatically maintaining sidetone in the normal range and reducing excessively high receive levels.